Juliet assures Sophie that Sidney is in fine health. She hosted a supper party for him last night and cooked the entire meal herself. Sidney is having a great time with Isola. Apparently she asked him last night when he was going to marry Juliet, and he informed her that he's gay. When he assured Isola that Juliet has always known, she kissed him, said that he's just like Booker, promised to keep it a secret, and started talking about Oscar Wilde's plays. He wants to get her a cuckoo clock as a gift. In a postscript, Juliet also tells Sophie about her horrendous phone call with Mark in which she again refused to marry him.
Isola's reaction to hearing that Sidney is gay suggests that while she may promise to keep it a secret, Sidney shouldn't necessarily trust her given that she outs Booker in the process. However, this does give Isola valuable information about the rest of Juliet's chosen family so that Isola can more effectively puzzle out where Juliet's romantic loyalties lie.